Achieving better control over mechanical and process properties so that adhesives can be tailored for specific, highly demanding end-use applications such as packaging, medical, and masking tapes is a continuing need in the pressure sensitive adhesive (PSA) art. Each end use application requires a different balance of properties.
Among the earliest polymers to provide a reasonable balance of the properties required for satisfactory PSA performance were the natural and synthetic rubbers. However, these PSAs had poor aging properties, and the need to eliminate ethylenically unsaturated groups from the polymer backbone of rubber adhesives to improve their oxidative stability was eventually recognized.
With the discovery of Ziegler-Natta (ZN) catalysts, polymerization of .alpha.-olefin monomers to high molecular weights became possible. Homopolymers of C.sub.6 to C.sub.10 .alpha.-olefins were naturally tacky, had low toxicity, good aging properties, and favorable environmental stability (i.e., low oxidation). These homopolymers were also chemically inert, resistant to plasticizer migration, and relatively inexpensive. They were, therefore, good candidates for PSA applications. However, their poor cohesive strength meant that they lacked the characteristics necessary for use at elevated temperatures.
Radiation curing frequently has been used to crosslink the polymeric component of adhesives in attempts to increase the cohesive strength of coated adhesive films. The results have not been completely satisfactory because degradation (i.e., chain scission) competes with the desired crosslinking reaction. Therefore, in some PSA systems, especially those formulated from polymers containing propylene, radiation curing leads to a loss of cohesive strength and shear adhesion.
Adhesives derived primarily from C.sub.6 or larger .alpha.-olefins are well known in the art. In U.S. Pat. No. 3,542,717, poly(.alpha.-olefin) adhesive compositions comprising mixtures of polyolefin copolymers derived from olefin monomers with different molecular weights (i.e., olefins with 11 to 20 carbon atoms, and different .alpha.-olefins with 4 to 20 carbon atoms) are described. The resulting adhesive has high shear and peel adhesion at room temperature but is unacceptable for most high temperature applications due to low cohesive strength and shear adhesion.
U.S. Pat. No. 3,635,755 describes PSA polyolefins suitable for use as single component PSAs for surgical tapes that are substantially non-allergenic. Such adhesives can be made from homopolymers of the C.sub.6 to C.sub.11 .alpha.-olefins or from interpolymers of C.sub.2 to C.sub.16 .alpha.-olefins having an intrinsic viscosity of 1.5 to 7.0, a Williams plasticity of 0.8 to 4.0, and an acetone/heptane soluble fraction of less than 25% by weight. Tapes made from these adhesives have low shear adhesions that facilitate non-irritating removal from the skin. However, their low shear adhesion makes these materials unsuitable for masking or packaging tape applications.
U.S. Pat. Nos. 3,954,697 and 4,178,272 describe hot melt adhesives, useful for sealing polyethylene films, derived from copolymers of propylene and C.sub.6 to C.sub.10 .alpha.-olefins. The '697 patent teaches that, although C.sub.6 or C.sub.8 .alpha.-olefin copolymers are permanently tacky, they have poor cohesive strength and, consequently, lack shear adhesion. Therefore, higher .alpha.-olefins must be copolymerized with propylene to meet "the critical property requirements for a pressure sensitive adhesive material." One such a property, cohesive strength, is discussed at col. 1, lines 34-48. The copolymer must contain more than 40 mole percent propylene, especially if static shear is to exceed 1000 minutes.
The '272 patent describes compositions comprising a ZN catalyst system and discloses process conditions useful in preparing the tacky copolymers required for PSAs. Also disclosed are stereoregulating, three-component ZN catalyst systems that produce high molecular weight, crystalline block copolymers unsuitable for PSA applications (col. 2, lines 39-65) but which are said to be useful in sealing bags, cartons, and on an adhesive for floor tiles.
U.S. Pat. No. 4,288,358 describes hot melt adhesive compositions containing terpolymers of propylene, either 1-butene or 1-pentene, and C.sub.6 to C.sub.10 .alpha.-olefins; tackifying resins; and plasticizing oils. Efforts to simultaneously optimize the adhesive and process properties of these compositions are described.
The preceding references teach optimized processing conditions for preparation of hot melt adhesives that are readily coatable or extrudable at the melt temperature. However, these hot melt adhesives have poor PSA properties. In particular, they have poor adhesion under shear. High temperature masking tapes used in the automotive industry must have good shear adhesion at elevated temperatures.
Attempts to radiation crosslink these poly(.alpha.-olefin) adhesives that contain propylene led to degradation of PSA properties (e.g., shear adhesion).
U.S. Pat. Nos. 2,933,480 and 3,933,769 describe a process for copolymerizing mixtures of .alpha.-olefins and non-conjugated diolefins to yield sulfur-vulcanizable unsaturated elastomers described as "rubber-like" or "rubbery".
U.S. Pat. No. 4,990,585 discloses a hexene- 1/4-methylpentene-1 random copolymer, plus a process for producing the copolymer, which is said to be useful as a rubber vibration insulator, a modifier for plastics and rubbers, and an adhesive.
U.S. Pat. No. 5,112,882 describes adhesive tapes that include radiation curable poly(.alpha.-olefin) PSA compositions with superior cohesive strength and shear adhesion. These compositions, which optionally can contain tackifiers, have desirable peel adhesion, shear strength, and stability against oxidative degradation at low and moderate temperatures.